A common occurrence in older persons is compression fractures of the vertebrae, causing both pain and a shortening (or other distortion) of stature. One common treatment is vertebroplasty, in which cement is injected into a fractured vertebra. While this treatment fixes the fracture and reduces pain, it does not restore the vertebra and person to their original height. Another problem is that the injected cement may be injected out of the vertebra or may migrate out through cracks in the vertebra. This may cause considerable bodily harm.
Another common treatment is kyphoplasty, in which the fracture is reduced, for example by first inflating a balloon inside the vertebra and then injecting a fixing material and/or an implant. The problem of cement migration is reduced, but not avoided, as a lower pressure can be used to inject the cement.
Some fixing materials, such as polymethylmethacrylate (PMMA), emit heat and possibly toxic materials while setting. These may further weaken the bone and possibly cause the cement to loosen and/or the bone to fracture.
It has recently been suggested that some fixing materials, being harder than bone, induce fractures in nearby bones.
It is also known to use bone-like repair materials, such as a slurry of bone chips, which apparently do not induce such fractures. However, injecting such materials is difficult due to their viscosity. There have also been attempts to reduce cement migration by injecting more viscous cement, for example, during the doughing time and the beginning of polymerization. However, the injection methods suggested require higher pressures for the more viscous material. Also, some types of viscous materials, such as hardening PMMA, have a small workability window at high viscosities, as they harden very quickly once they reach a high viscosity. This has generally prevented very viscous materials and the associated very high pressures from being used. One possible reason is that as pressures increase, the physician is prevented from receiving feedback on the resistance of the body to the injection of the cement. Thus, over-injection can easily occur.
Another way of increasing viscosity for injection is increasing a monomer-powder concentration ratio (MPR). However, it should be noted that increasing a cement's MPR can lead to marked drops in some of its mechanical properties, such as elastic modulus, yield strength and ultimate strength.
US patents and U.S. Pat. Nos. 4,969,888, 5,108,404, 6,383,188, 2003/0109883, 2002/0068974, U.S. Pat. Nos. 6,348,055, 6,383,190, 4,494,535, 4,653,489 and 4,653,487, the disclosures of which are incorporated herein by reference describe various tools and methods for treating bone. The present application is related to U.S. application Ser. No. 09/890,172 (now U.S. Pat. No. 7,621,950) filed on Jul. 25, 2001 and U.S. application Ser. No. 09/890,318 (now U.S. Pat. No. 7,097,648) filed on Jul. 25, 2001, the disclosure of which are incorporated herein by reference.